Let's roll up our sleeves and solve this. We have 139 and we're going to multiply it by 144, essentially adding 139 to itself 144 times.
Step 1: 0 + 139 = 139
Step 2: 139 + 139 = 278
Step 3: 278 + 139 = 417
Step 4: 417 + 139 = 556
Step 5: 556 + 139 = 695
Step 6: 695 + 139 = 834
Step 7: 834 + 139 = 973
Step 8: 973 + 139 = 1112
Step 9: 1112 + 139 = 1251
Step 10: 1251 + 139 = 1390
Step 11: 1390 + 139 = 1529
Step 12: 1529 + 139 = 1668
Step 13: 1668 + 139 = 1807
Step 14: 1807 + 139 = 1946
Step 15: 1946 + 139 = 2085
Step 16: 2085 + 139 = 2224
Step 17: 2224 + 139 = 2363
Step 18: 2363 + 139 = 2502
Step 19: 2502 + 139 = 2641
Step 20: 2641 + 139 = 2780
Step 21: 2780 + 139 = 2919
Step 22: 2919 + 139 = 3058
Step 23: 3058 + 139 = 3197
Step 24: 3197 + 139 = 3336
Step 25: 3336 + 139 = 3475
Step 26: 3475 + 139 = 3614
Step 27: 3614 + 139 = 3753
Step 28: 3753 + 139 = 3892
Step 29: 3892 + 139 = 4031
Step 30: 4031 + 139 = 4170
Step 31: 4170 + 139 = 4309
Step 32: 4309 + 139 = 4448
Step 33: 4448 + 139 = 4587
Step 34: 4587 + 139 = 4726
Step 35: 4726 + 139 = 4865
Step 36: 4865 + 139 = 5004
Step 37: 5004 + 139 = 5143
Step 38: 5143 + 139 = 5282
Step 39: 5282 + 139 = 5421
Step 40: 5421 + 139 = 5560
Step 41: 5560 + 139 = 5699
Step 42: 5699 + 139 = 5838
Step 43: 5838 + 139 = 5977
Step 44: 5977 + 139 = 6116
Step 45: 6116 + 139 = 6255
Step 46: 6255 + 139 = 6394
Step 47: 6394 + 139 = 6533
Step 48: 6533 + 139 = 6672
Step 49: 6672 + 139 = 6811
Step 50: 6811 + 139 = 6950
Step 51: 6950 + 139 = 7089
Step 52: 7089 + 139 = 7228
Step 53: 7228 + 139 = 7367
Step 54: 7367 + 139 = 7506
Step 55: 7506 + 139 = 7645
Step 56: 7645 + 139 = 7784
Step 57: 7784 + 139 = 7923
Step 58: 7923 + 139 = 8062
Step 59: 8062 + 139 = 8201
Step 60: 8201 + 139 = 8340
Step 61: 8340 + 139 = 8479
Step 62: 8479 + 139 = 8618
Step 63: 8618 + 139 = 8757
Step 64: 8757 + 139 = 8896
Step 65: 8896 + 139 = 9035
Step 66: 9035 + 139 = 9174
Step 67: 9174 + 139 = 9313
Step 68: 9313 + 139 = 9452
Step 69: 9452 + 139 = 9591
Step 70: 9591 + 139 = 9730
Step 71: 9730 + 139 = 9869
Step 72: 9869 + 139 = 10008
Step 73: 10008 + 139 = 10147
Step 74: 10147 + 139 = 10286
Step 75: 10286 + 139 = 10425
Step 76: 10425 + 139 = 10564
Step 77: 10564 + 139 = 10703
Step 78: 10703 + 139 = 10842
Step 79: 10842 + 139 = 10981
Step 80: 10981 + 139 = 11120
Step 81: 11120 + 139 = 11259
Step 82: 11259 + 139 = 11398
Step 83: 11398 + 139 = 11537
Step 84: 11537 + 139 = 11676
Step 85: 11676 + 139 = 11815
Step 86: 11815 + 139 = 11954
Step 87: 11954 + 139 = 12093
Step 88: 12093 + 139 = 12232
Step 89: 12232 + 139 = 12371
Step 90: 12371 + 139 = 12510
Step 91: 12510 + 139 = 12649
Step 92: 12649 + 139 = 12788
Step 93: 12788 + 139 = 12927
Step 94: 12927 + 139 = 13066
Step 95: 13066 + 139 = 13205
Step 96: 13205 + 139 = 13344
Step 97: 13344 + 139 = 13483
Step 98: 13483 + 139 = 13622
Step 99: 13622 + 139 = 13761
Step 100: 13761 + 139 = 13900
Step 101: 13900 + 139 = 14039
Step 102: 14039 + 139 = 14178
Step 103: 14178 + 139 = 14317
Step 104: 14317 + 139 = 14456
Step 105: 14456 + 139 = 14595
Step 106: 14595 + 139 = 14734
Step 107: 14734 + 139 = 14873
Step 108: 14873 + 139 = 15012
Step 109: 15012 + 139 = 15151
Step 110: 15151 + 139 = 15290
Step 111: 15290 + 139 = 15429
Step 112: 15429 + 139 = 15568
Step 113: 15568 + 139 = 15707
Step 114: 15707 + 139 = 15846
Step 115: 15846 + 139 = 15985
Step 116: 15985 + 139 = 16124
Step 117: 16124 + 139 = 16263
Step 118: 16263 + 139 = 16402
Step 119: 16402 + 139 = 16541
Step 120: 16541 + 139 = 16680
Step 121: 16680 + 139 = 16819
Step 122: 16819 + 139 = 16958
Step 123: 16958 + 139 = 17097
Step 124: 17097 + 139 = 17236
Step 125: 17236 + 139 = 17375
Step 126: 17375 + 139 = 17514
Step 127: 17514 + 139 = 17653
Step 128: 17653 + 139 = 17792
Step 129: 17792 + 139 = 17931
Step 130: 17931 + 139 = 18070
Step 131: 18070 + 139 = 18209
Step 132: 18209 + 139 = 18348
Step 133: 18348 + 139 = 18487
Step 134: 18487 + 139 = 18626
Step 135: 18626 + 139 = 18765
Step 136: 18765 + 139 = 18904
Step 137: 18904 + 139 = 19043
Step 138: 19043 + 139 = 19182
Step 139: 19182 + 139 = 19321
Step 140: 19321 + 139 = 19460
Step 141: 19460 + 139 = 19599
Step 142: 19599 + 139 = 19738
Step 143: 19738 + 139 = 19877
Step 144: 19877 + 139 = 20016

So, 139*144 = 20016